The present invention relates to an information storage device which uses a magnetic disk device, an optical disk device or any other rotary recording medium. More specifically, the present invention relates to improvements in a head positioning mechanism for detecting information in such devices.
Information storage devices such as magnetic disk devices, optical disk devices and magnetooptical disk devices have heretofore been used as external memory devices for electronic computers and image processors. Among such devices, information storage devices have been known in which a disk of a medium for recording information is rotated, and a head is moved to detect information on the disk, in order that the detection head writes information onto any position on the disk surface or reads information from any position on the disk surface.
FIG. 7 shows the structure of, for instance, such a magnetic disk device. In this magnetic disk device, the head is positioned by linearly moving the carriage and by moving the head in the radial direction of the disk that is rotating. The disks a are fitted and secured to a spindle b which is held by spindle bearings d fitted to a housing c. A spindle motor e is installed on the housing and has its shaft coupled to the spindle. The heads f are mounted on guide arms g via gimbals. The guide arms g are installed on a carriage h. Rollers in the carriage h roll on the rails in the housing, enabling the heads to move in a radial direction of the disks. The driving is accomplished by an actuator that has a coil i and a magnet. The coil i is mounted on the carriage h, and the magnet is secured to the housing c together with a yoke j that carries the magnet. The carriage attains the aforementioned linear motion due to the electromagnetic action caused by the coil and the magnet.
In such a magnetic disk device, when the carriage h is moved by the electromagnetic action caused by the coil and the magnet, the magnet and the yoke j supporting the magnet receive the reaction force. Since the yoke j is directly secured to the housing c, the reaction force is directly transmitted to the housing c and is further transmitted to the spindle b via the spindle bearings d.
Since the spindle bearings d have a relatively small rigidity, a spring-mass system is established in which the spindle bearings d serve as springs and the spindle b serves as a mass, whereby the spindle b vibrates together with the disks a (hereinafter this phenomenon will be called spindle vibration).
Since the spindle bearing d has almost no viscosity, the vibration crest becomes high at the resonance frequency of the spring-mass system, whereby the carriage h is no longer capable of sufficiently following the vibration, resulting in the occurrence of a head location error. FIG. 8 shows the relationship between the spindle vibration and the positioning precision of the head, showing the vibration frequency as the abscissa and the displacement of the head, i.e., the head location error as the ordinate.
In the above-mentioned magnetic disk device, in other words, when the excitation frequency component due to the reaction force of the actuator comes to agree with a proper vibration frequency of the spindle, the resonance crest (curve A) becomes high since the spindle bearing has a small viscosity. Additionally, the positioning precision of the head decreases.